1. Field of the Invention
This invention relates to a device for controlling ignition timing in an internal combustion engine, more specifically to a device for controlling ignition timing in an internal combustion engine on the basis of the detected state of combustion in the combustion chambers of the internal combustion engine.
2. Description of the Prior Art
It is well known that the output of the internal combustion engine becomes maximum when ignition is controlled to occur so that the crankshaft angle at which the cylinder pressure is maximum (this angle being referred to as .theta.pmax in this specification) is in the vicinity of 15.degree. ATDC (after top dead center). An example of a technique for maximizing engine output in this manner can be found in Japanese Unexamined Patent Publication No. 59(1984)-39974. In the technique disclosed by this prior art, one pressure sensor is provided for every two cylinders and the output of each pressure sensor is sent through a low frequency band pass filter to a sample hold circuit and then to an A/D converter where it is digitized once per degree of crankshaft rotation under the control of a timing signal from a crankshaft angle sensor. The digitally converted signal is input to the CPU (central processing unit) of a microcomputer which uses it to determine the maximum cylinder pressure angle, whereafter the deviation from the target maximum-pressure angle (ATDC 10.degree.-15.degree.) is corrected in angular increments of predetermined magnitude. Simultaneously, for the purpose of discriminating knocking on the basis of the output from the pressure sensor, this output is also passed through a high frequency band pass filter connected in parallel with the aforesaid low frequency band pass filter. The signal from this high frequency band pass filter is integrated in a circuit for producing a reference signal for knocking determination and the so-obtained knocking reference signal is sent to a comparator where it is compared with the output of the aforesaid filter. The result of this comparison is sent to the CPU where it is determined whether or not knocking is present. When knocking is found to be present, the ignition timing is retarded.
This prior art has the defect of requiring separate circuits for detecting the maximum cylinder pressure angle and the presence/absence of knocking, which necessitates the use of complicated and expensive device. Also, as the circuit arrangement is complex, the reliability of the operation is correspondingly low.
Moreover, although the ignition timing is regulated in response to the deviation between the maximum cylinder pressure angle and the target maximum-pressure angle, the size of the angular increments by which this regulation is made is always the same no matter how large or small the deviation may be. As a result, when the deviation is large, considerable time is required before the target maximum-pressure angle is reached, which tends to lower the engine output. It is not possible to overcome this problem simply by carrying out the regulation in large increments because this would result in abrupt changes in the ignition timing which would degrade the drivability of the vehicle driven by the engine.
At any rate, in this prior art reference, neither the regulation for elimination of knocking nor the regulation for elimination of the deviation in maximum cylinder pressure angle is carried out separately for each cylinder; both are carried out for all cylinders together. As a result, when knocking occurs, be it only in one cylinder, all of the cylinders including those in which knocking has not occurred are subjected to ignition timing delay. This tends to reduce the engine output unnecessary.